1. Field of the Invention
This invention is generally related to the field of geophysical prospecting and more particularly to a method and apparatus for prospecting for buried mineral deposits such as uranium and thorium which is based upon the detection of radioactive decay products emanating from such deposits and exiting from the earth's surface into the atmosphere.
2. Description of the Prior Art
It is well-known that mineral deposits such as uranium and thorium give rise to emanations of radioactive gaseous decay products such as radon which migrate upwardly through the rock and soil of the earth by diffusion and transport.
A method of detection of the presence of such deposits consists in drilling holes in a geologically favorable area, evacuating a quantity of gas from the substrata region under consideration, and thereafter measuring by known techniques the quantity of a particular radioactive isotope present in the gas. This method suffers from the drawback, however, that even in the vicinity of a radioactive mineral deposit the gas sample collected from a given well may be a poor indicator. It is known that the fractures or channels characteristic of crustal rock provide high speed, directional transport paths for radioactive gas, for example, radon-222. However, a single bore hole may conceivably extend to a depth of several hundred feet without intersecting more than a small number of such microfractures. Conversely, such a bore hole may fortuitously intersect a very large number of such paths. In either case, the readings taken from such bore holes may not be representative of the average gas concentration in the vicinity and thus may not accurately predict the location and probable contour of a buried radioactive deposit such as uranium.
Another method of detecting the presence of subsurface radioactive deposits consists in the detection and tracking of windblown gaseous clouds in the atmosphere arising from radioactive decay products diffusing upwardly through the earth. The airborne gas and decay products thereof can allegedly be detected by means of observations over the ground surface and at points removed from the deposits by significant distance. This method is exemplified by U.S. Pat. No. 3,609,363 and depends for its success upon "the application of meteorological knowledge concerning cloud travel and detection, to govern the observational regime and the interpretation of measurements." Windblown gas decay products, however, are generally regarded as weak indicators of the presence of subsurface radioactive deposits. There are many atmospheric anomalies which are difficult, if not impossible, to predict. For example, radon, along with other gases, tends to be trapped in inversion layers. If the inversion clears up, the gas will disperse. Furthermore, radon clouds may travel extremely long distances in the air from the location of an ore body before they are detected.
In an effort to employ the technique described, the cited patent proposes, after detection and general localization of an area of interest, to outline the ore body itself. This is said to be done "by capping the emanating soil surface to trap the gaseous products at a succession of points over the area in question." There is no indication given of the size, areal extent, shape, material, or method of positioning the proposed collection containers. As will become apparent from the description to follow, these features may influence directly the value of the collected gas samples as an indicator of the presence of radioactive isotopes and the contour of the buried deposit or deposits responsible for the presence of such decay products.
Still another surface exploration technique for locating buried ore bodies typically involves placement of small inverted cups in shallow covered holes on the order of 2 to 3 feet deep in the vicinity of interest. Sensitive film may be positioned within the cups and left for a period of time to arrive at equilibrium conditions. For the same reasons referenced above, the sampling of gas concentration in the soil in accordance with such a method may not predict with consistent accuracy the presence or contour of buried ore bodies.